Hello dev ,

Please try with the below trigger :
 

trigger OpportunityTrigger on Opportunity (after insert, after update) {
    // List to hold follow-up tasks to be created
    List<Task> followUpTasks = new List<Task>();

    // Map to store task Ids and their corresponding Opportunity Ids
    Map<Id, Id> taskToOpportunityMap = new Map<Id, Id>();

    // Check if the trigger is fired after an opportunity is created
    if (Trigger.isAfter && Trigger.isInsert) {
        for (Opportunity opp : Trigger.new) {
            // Create a follow-up task for each new opportunity
            Task followUpTask = new Task(
                Subject = 'Follow-Up on Opportunity: ' + opp.Name,
                Description = 'Follow up on the opportunity with an expected close date of ' + opp.CloseDate,
                WhatId = opp.Id,
                Status = 'Not Started',
                Priority = 'Normal'
            );
            followUpTasks.add(followUpTask);
            taskToOpportunityMap.put(followUpTask.Id, opp.Id);
        }
        // Insert the follow-up tasks
        insert followUpTasks;
    }

    // Check if the trigger is fired after a task is updated
    if (Trigger.isAfter && Trigger.isUpdate) {
        for (Task task : Trigger.new) {
            Task oldTask = Trigger.oldMap.get(task.Id);
            // Check if the task status is changed from 'Not Started' to 'Completed'
            if (task.Status == 'Completed' && oldTask.Status != 'Completed') {
                // Update the parent opportunity status to 'Closed Won'
                Id parentOpportunityId = taskToOpportunityMap.get(task.Id);
                Opportunity parentOpportunity = new Opportunity(
                    Id = parentOpportunityId,
                    StageName = 'Closed Won'
                );
                update parentOpportunity;
            }
        }
    }
}

This trigger fires after the insert and after the update events of an Opportunity and Task objects. When an Opportunity is inserted, the trigger creates a follow-up Task associated with the Opportunity. When a Task's status is changed to 'Completed,' the trigger finds the parent Opportunity and updates its StageName to 'Closed Won.

Hope this helps !
Thank you.